Many prior art load control systems have been proposed measuring motor current. However, this parameter varies considerably with ambient temperature variations and furthermore it is difficult to adapt a motor in the field to such measurements. In general also the variations of current with load is not linear over a large enough range of load conditions and thus becomes difficult to use for precision indications of loading. Furthermore, the current characteristics of different size motors vary considerably so that custom designed circuitry is necessary for different motor sizes.
While it is known that motor slip speed is a good indicator of load conditions substantially linear over a large range of load conditions, that parameter has not been effectively used for simple load monitoring and control application. Thus, complex and costly electronic timing or counting circuits using precision type oscillators requiring temperature ovens or other precision type equipment have been necessary for measuring the small number of slip RPM such as 30 out of a high nominal rotating RPM such as 1800.